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The Insulation Lab

Fiberglass verses Cellulose - Which is Better

Jeff Aderholdt - Tuesday, March 27, 2012

Ford or Chevy? Coke or Pepsi? PC or Mac? Less filling, taste great? Over the years there have been many controversies and debates. In insulation, one of the biggest debates is over which is the better insulation material, fiberglass or cellulose. Well, which is REALLY better?


Just the Facts

Before we start, we need to remember that insulation science is a “science”. Because of that, we need to be careful not to allow emotion to overcome reason. This is easier said than done. There are always those spinning and skewing the information to serve their own objectives. One way we see this is when scientific studies get hijacked for political agendas, thus creating a fog that obscures and distorts the facts, and gets peoples passions to override their thinking.

A second way we see this is through marketing. With the objective of promoting their product over their competitors, advertisers emphasize their products good points and downplay their bad points (we usually call this “Spinning”), all while emphasizing their competitors bad points and downplaying their good points This is commonly referred to as “FUD”, creating Fear, Uncertainty & Doubt). The methods used can be overt, readily manifest, or covert, deliberately hidden. Their objective is simple. Buy our product, not our competitors. And though it's important to produce a good product, to market successfully something more is needed.

To illustrate this, consider the example of one of the most successful companies of modern times, Apple. Early in the company's history, they knew they had a good product, but with companies like IBM looming over them, they knew that marketing was very important. So they created a company position to do just that. That position was called “Chief Evangelizer”. His job was not just to promote the products, but to spread the gospel of Apple. True, a good product is important, but to really capture and hold a marketplace, you need to get emotions involved, and what emotion is more powerful than religious fervor. The result is that Apple has attained a level that few companies ever have. They have successfully marketed not just a product, but a life style. So, what does this have to do with insulation?

Separating Facts from Marketing

The insulation industry is no different than any other industry. When it comes to information on various insulation products and techniques, there is a lot available. Included in this will be information that is skewed and distorted, is spun and full of FUD, for marketing or other objectives. How do we know what to believe? Here's a few tips.

1. Consider the Source. Who is providing the information and do they have a vested interest? In researching for this article, I found a site for a “construction professional” who was writing on the same subject. To support some statements that appeared to me to be covertly biased, he referred to information that he received from NAIMA (North American Insulation Manufacturers Association). On the surface this seems like a good authoritative source. But look deeper! With a little research you learn that NAIMA is an association for fiberglass manufacturers. Since NAIMA has an obvious vested interest in the matter, it calls into question the credibility of the information, as well as the credibility of the person using this information to support his conclusions. The same would hold true for information from any other biased association, such as the CIMA (Cellulose Insulation Manufacturers Association).

2. Read Carefully. What does it say, and what doesn't it say? Is the information complete, or are there subtle gaps in the information? Is some key information glossed over? In the same information that the “construction professional” received from NAIMA, an incident was related where during a remodel job, the electrician set his trouble light in the cellulose insulation and went to lunch. As a result, the house caught on fire and cellulose was to blame. But look deeper! What wasn't said? First off, it was a remodeling job. So, how old was the cellulose insulation? Back in the early days, some cellulose manufacturers used little or no fire retardants. To use this to say that modern cellulose is a fire hazard would be, well, deceptive. Also, what kind of trouble light was left in the cellulose? Was it one of the newer 500 watt halogen bulbs? One of those would set the lumber on fire! Now, what was glossed over? The electrician put his light where it didn't belong, in the insulation! So, is this a case of bad materials or incompetence? Another thing to watch for in reading is an overuse of adjectives like “very”, “highly”, extremely, and so forth. Though there are legitimate usages for such descriptive words, these are the tools of those skilled in spin and FUD.

3. Avoid Anecdotal Information. Urban legends, old wives tales, my uncle knows a guy who met a guy that …. interestingly, in the same information that the “construction professional” received from NAIMA, it was prefaced as “ an anecdote”. The problem with information of this type is, it is hard to verify for accuracy, and it very rarely gives the entire story. There is no place in science, including insulation science, for such suspect information.


What are the Facts – What do we Know

So, what do we know, first about fiberglass? Fiberglass insulation has been around a long time and it continues to be the most common insulation used in construction. Depending on the application, it produces an insulation value from about R-2.0 to R-3.8 per inch. The thin strands of fiberglass make a bunch of little air pockets, and the trapped air serves as the insulation. This also makes fiberglass relatively light. Since it's main ingredient is glass, it is made of sand. In recent years, fiberglass manufacturers have made more use of recycled glass. Since it is inorganic, it doesn't decay and it doesn't burn, although the same cannot be said about the added binders and coloring.

Now, what do we know about cellulose? Cellulose insulation has also been around for a long time. Wasps have been using cellulose to build and insulate their nests for as long as there have been wasps. It produces an insulation value of about R-3.5 to R-3.8 per inch. It is heavier and more dense than fiberglass. The main ingredient is recycled paper. Since it is organic, it needs to be treated to make it resistant to fire.

Specific areas of Comparison


Either materials will provide adequate R-value. The real question is how well it maintains it. There are some applications with fiberglass that create a situation that make it difficult to maintain the desired R-value. The first one is the use of batting in wall cavities. It is virtually impossible to install batting in a wall without having voids. Enough voids will allow the air to circulate within the wall cavity. The result is a decrease in the functional R-value. The second area of concern is with blown fiberglass in an attic. Blowing fiberglass has a tendency to be real light and fluffy. This means that the trapped air that is creating the insulation doesn't stay trapped, especially under adverse weather conditions. One study done several years ago found that a house heated to 70°F with it's attic insulated with blown fiberglass would loose 50% of it's functional R-Value at -18°F.

Cellulose insulation, even under extreme situations, maintains a consistent R-value. This is mostly because of the density of cellulose insulation. Some studies show that performance even increases slightly in cold weather.

Air Infiltration/Exfiltration

In recent years, the importance of controlling air infiltration/exfiltration has become apparent. This is job one in building an energy efficient house. This is an area where fiberglass fails and cellulose excels. Again this goes to the dense nature of cellulose insulation. Not only does the density of cellulose restrict air movement within itself, thus maintaining a consistent R-Value, it restricts air movement through it, reducing air infiltration/exfiltration. The dense pack cellulose technique has been show to virtually eliminate air infiltration/exfiltration. A Princeton University study of a group of homes with blown in cellulose insulation in the walls had an average of 24.5% reduction of air infiltration compared to fiberglass insulation.  A study by the Leominster MA Housing Project for the Elderly found that a building with blown cellulose had 40% lower leakage compared to one insulated with fiberglass. So, though not the primary air barrier, cellulose proves to be an excellent supporting player.



Fire Safety

This area really needs to be divided into 2 areas.
1. The materials resistance to fire (Does it burn?)
2. The materials effect as a fire barrier (Will fire move through it?)

Fire Safety – Fire Resistance

Cellulose is made from organic fibers, which are combustible. Chemicals are added to make cellulose fire resistant. Fiberglass in made from inorganic mineral fibers, which are inherently non-combustible. But, added to fiberglass are things to glue it together and give it color (what, did you really think fiberglass is naturally pink or yellow?), and these will burn.

So what does this all mean? Neither are fire proof. Neither should be put in direct contact with excessive heat. Building codes address this requiring an air space between insulation, either fiberglass or cellulose, and high heat sources, such as metal chimneys and non-IC light fixtures. That being said, most insulation contractors as a matter of prudence, will place fiberglass in contact with questionable areas, just in case. The reason for fiberglass is because, 1. it is mostly inorganic, and therefore less likely to burn, and 2. fiberglass allows any built up hot air to move out of it and dissipate.

Fire Safety – Fire Barrier

The function of a fire barrier is to keep a fire from moving through it, preventing a fire from spreading. You will commonly see this in large attics where a drywall fire wall is installed. If a fire occurs on one side of the firewall, it needs to burn through this wall before it can spread. The result is a smaller, more contained fire.

If someone is building a wall, floor or ceiling to serve as a fire barrier only, it may be best to leave it un-insulated. This allows the heat to dissipate away from the wall material, thereby increasing the time to failure. Of course, leaving walls, floors and ceilings empty is not always a option. We have other concerns, such a heat and sound control, and this requires insulation.

When fiberglass or cellulose is added, there is a reduction in the barrier time to structural failure. Though cellulose appears to have a slight advantage, it is not enough to be a deciding factor for choice of material. The tests, though, did reveal something interesting. Though many of the tests are looking at structural failure, as in how long before a weight bearing wall will collapse, some tests also looked at how long before the flame passed through. Those tests showed a definite advantage for cellulose, in some cases over 30% over an empty cavity (In the same test, fiberglass reduced performance by 20%).

So what does this all mean? Ultimately very little in regards to choice of materials. If the objective is to build a high performing firewall, there are more important considerations than the type of insulation used.


As with all choices we make, in one way or another it comes down to economics. We need to compare costs with benefits. Am I getting what I pay for?

Fiberglass is made from, well, sand and sand is cheap. The biggest manufacturing cost is energy. It takes a lot of heat to melt the sand into glass, and this is reflected in the cost. Use of more recycled glass has helped, still, as energy costs go up, we can expect the cost of fiberglass to go up proportionally. Cellulose is made primarily of recycled materials and take very little additional cost to manufacture. In recent years, the most noticeable factor in the cost of cellulose is the cost of transportation.

What does this mean to the consumer? For most applications, cellulose gives the most for the money spent. In an open attic, cellulose can be installed faster and less expensively than fiberglass, either blown or batt. Though it is true that, because of additional labor, using cellulose in the walls (See Dense-pack Cellulose) is slightly more expensive than using fiberglass batting of a comparable R-Value, the benefits of the reduction in air infiltration more than compensate for the initial cost. Though in other areas, the difference between fiberglass and cellulose may be negligible, when it come to cost, cellulose has the clear advantage.

Conclusion – Which is Really Better?

The simple truth is, both, and sometimes neither. Both have there place. In some areas, one may have an advantage, but in most cases, it is not enough to base your decision on. This is where your insulation contractor comes in (See “How to pick an Insulator”). In general:

1. Attic insulation - don't batt what you can blow, either fiberglass or cellulose. In an attic, when you consider cost vs. benefits, it is hard to beat cellulose. Still there are some situations that call for fiberglass, as in if there is the potential for excessive heat. Fiberglass may also be better if there is a concern over weight, but if weight is a concern, the biggest issue really isn't the type of insulation being used. Sometimes fiberglass batting can be used to stabilize blown insulation, perhaps on a steep vaulted ceiling.

2. Wall insulation. The air infiltration factor gives the edge again to cellulose (SeeDense-pack cellulose). The slight up charge for dense-pack is compensated for by the added energy savings and comfort, making cellulose a value.

3. Box sill. This is an area where neither fiberglass or cellulose may be the best choice. Though a lot more expensive, spray-foam is probably a better choice. Spray-foam will seal up the cracks and reduce the potential for moisture issues. Still, if you have a builder willing to do a little creative framing, dense-pack cellulose can give comparable performance to spray foam and do it more cost-effectively (I plan on discussing this option further in the future. Update 12/24/2012 - see Box Sill Insulation - To Foam or Not To Foam).

4. Insulating over masonry foundation walls. Again, this is an area where neither fiberglass or cellulose may be the best choice. Probably a better choice is installing sheet foam, such as polystyrene foam or a fire-rated urethane foam board where it will be left exposed. This will help minimize the potential for condensation, moisture and mold problems.

At NTC, we have worked with a variety of techniques and with a variety of materials. We know what will work best for your home. Give us a call and ask us, or have your building contractor give us a call. Let us show you why NTC is the best choice for all your insulation needs.
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